Embodiments of the invention relate generally to structures and methods for packaging RF filters such as surface acoustic wave (SAW) filters, bulk acoustic wave (BAW) filters and SAW resonators and, more particularly, to an embedded package structure having one or more SAW filters, BAW filters and/or SAW resonators integrated therein.
RF interference has always been an inhibitor of communications, requiring designers to take such interference into account when designing devices that employ wireless communication. In addressing issues of RF interference, today's wireless devices must not only reject signals from other services but from themselves, too, as the number of bands packed inside each device increases. A high-end smartphone or tablet must, for example, filter the transmit and receive paths for 2G, 3G, and 4G wireless access methods in up to 15 bands, as well as Wi-Fi, Bluetooth and the receive path of GPS receivers.
As part of the filtering process, signals in the receive paths must be isolated from one another and other extraneous signals must also be rejected. In order to provide adequate filtering, the wireless device must thus employ one or more RF filters for each frequency band allowed for consumer mobile communication. As there are more than a half dozen frequency bands utilized for communication, it is thus common for the wireless device to require the use of several tens of filters, thus resulting in packing density challenges. These filters are typically in the form of surface acoustic wave (SAW) filters, bulk acoustic wave (BAW) filters, or SAW resonators. In a basic SAW filter 100, such as illustrated in FIG. 1, an electrical input signal is provided to SAW filter via electrical ports (i.e., I/O pads) 102, with the electrical input signal being converted to an acoustic wave by interleaved metal interdigital transducers (IDTs) 104 created on a piezoelectric substrate 106, such as quartz, lithium tantalite (LiTaO3) or lithium niobate (LiNbO3). SAW filters combine low insertion loss with good rejection and can achieve broad bandwidths with SAW filters being well suited for up to about 1.5 GHz such that they are often used in 2G receiver front ends and in duplexers and receive filters. In a basic BAW filter 110, such as illustrated in FIG. 2, metal patches 112, 114 are formed/provided on top and bottom sides of a quartz crystal substrate 116 to excite acoustic waves responsive to an electrical input signal provided thereto via electrical ports 118, with the acoustic waves bouncing from the top to bottom surface (i.e., propagate vertically) to form a standing acoustic wave. The frequency at which resonance occurs is determined by the thickness of the substrate 116 and the mass of the electrodes 112, 114, with BAW filters being well suited for high frequency applications such that they are often used in 3G and 4G applications.
In existing wireless devices, each RF filter (i.e., each SAW/BAW filter/SAW resonator) included in the device is assembled separately into its own ceramic, metal sealed package—with such packaging being necessary because the acoustic wave in the SAW filter often propagates along or very near the surface, such that the SAW filter is generally very sensitive to surface conditions and requires protection. FIGS. 3A to 3E illustrate a stepwise conventional fabrication technique of a SAW filter chip package. With reference to FIG. 3A, a wafer (not shown) having plural SAW filter chips is divided into individual SAW filter chips 120, and a substrate 122 having plural mounting portions corresponding to SAW filter chips 120 is provided. Protectors 124 are attached to a lower side of the SAW filter chip 120 to form an air gap for protecting the surface of the SAW filter 120, and bumps 126 for flip chip bonding are attached to an upper side of the substrate 122.
Referring to FIG. 3B, each SAW filter chip 120 is mounted on a mounting portion of the substrate 122, and the SAW filter chip is electrically and mechanically connected to a wiring portion of the substrate 122 by flip chip bonding. In alternative embodiments, it is recognized that the SAW filter chip 120 could also be wire bonded to connections on the substrate 122. As shown in FIG. 3C, underfills 128 are then filled into a space between the substrate and the SAW filter chip. When underfills 128 are filled between the substrate 122 and the SAW filter chip 120, an active region positioned on a lower side surface of the SAW filter chip 120 is protected by the air gap formed by protectors 124.
Referring to FIG. 3D, fillets 130 are formed in order to improve step-coverages of sides of SAW filter chips. Fillets 130 are composed of an insulating material, and give a gentle slope to sides of the SAW filter chip 120 having the form of a stepped pyramid, so that a metal layer can be easily formed on the SAW filter chip. After the fillet 130 is formed, a metal shield layer 132 is formed on the outer wall of the SAW filter chip 120, as shown in FIG. 3E. To secure reliability of the SAW filter chip 120, an inner metal layer intercepting external electrical effects is formed on an upper side of the chip, and then an outer metal layer for preventing oxidation of the inner metal layer owing to exposure of the inner metal layer to the atmosphere is additionally formed on the inner metal layer.
As described above, according to a conventional method for fabricating SAW filter chip packages, SAW filter chips are packaged as individual chip units. That is to say, after plural chips on a wafer are divided into individual chips, each chip is mounted on a package substrate, electrically connected thereto via flip chip or wire bonding, an underfill material is provided to a space between each SAW filter chip and the package substrate, and the step of forming fillets or metal shield layers are conducted for individual chip unit. Accordingly, a method for fabricating SAW filter chip packages is very complicated and may require a certain amount of space clearance for bonding wire to connections of the package. Furthermore, as many filter packages are often assembled into a multi-chip module that also includes discrete components, the resulting modules may be large in size and expensive.
Therefore, it would be desirable to provide a method of forming a filter package that reduces the complexity and cost of fabrication. It would further be desirable for such a method to allow for formation of a filter package as part of an overall embedded filter module that also includes peripheral passive components, delay lines, antennas and switching matrices in the same package, with such co-packaging of all the components in one structure providing for lower cost plastic packaging, lower form factor, and higher integration, and packing density.